Generally, a composite dry vacuum pump has been used for evacuating the process chamber of a semiconductor manufacturing device, a display manufacturing device and the like, or for discharging gaseous substances and/or byproducts generated in the process chamber. A roots rotor, a screw rotor or combinations thereof is used in the dry vacuum pump as described above.
Currently, composite dry vacuum pumps include at least one roots rotor, having at least one lobe, and at least one screw rotor so as to keep the process chamber in a perfect vacuum and thereby reduce the power cost requirement. A roots rotor is used in inhaling and compressing the byproduct, including gaseous substances, generated in the process chamber, and a screw rotor is used in evacuating the gaseous substance and byproduct inhaled by the roots rotor to outside the dry vacuum pump. These rotors are operated in a sealed state to keep the process chamber in a vacuum.
In general, a partition wall is provided between a roots rotor end and a screw rotor end so that the byproducts do not hamper the rotor's rotation and can move smoothly from the roots rotor end to the screw rotor end. A representative example of these configurations is disclosed in U.S. Pat. No. 5,549,463, which is assigned entirely to Kashiyama Industry Co., Ltd.
According to this patent, as illustrated in FIG. 7, a dry vacuum pump comprises a pair of roots rotors 102, 103 and a pair of screw rotors 105, 106. The pair of roots rotors 102, 103 and the pair of screw rotors 105, 106 are rotated by a driving motor 104. The driving force generated by the driving motor 104 is transmitted entirely to the pair of roots rotors 102, 103 and the pair of screw rotors 105, 106 via three gears, that is, a drive gear 124, an idle gear 125 and a follower gear 127. A partition wall is provided between the pair of roots rotors 102, 103 and the pair of screw rotors 105, 106 so that the byproducts from the process chamber (not shown) are not transmitted directly to the pair of screw rotors 105, 106. In this conventional dry vacuum pump, shafts 114a, 114b respectively connected laterally to the middle of the pair of screw rotors 105, 106 penetrates through the partition wall 108, and the penetrated portions of the shafts are surrounded by a plurality of bearings 110a, 110b for smooth rotation of each shaft 114a, 114b. The opposing portions of each shaft 114a, 114b also are surrounded by a plurality of bearings 112a, 112b for the same reason. The entire disclosure of this patent is hereby incorporated by reference into the present application.
In the dry vacuum pump 100 disclosed in U.S. Pat. No. 5,549,463, since the partition wall 108 is formed between the pair of roots rotors 102, 103 and the pair of screw rotors 105, 106, the housing for these elements is divided into several portions, which requires increased effort and cost in manufacturing the pump.
A dry vacuum pump using a screw rotor in which a screw with a changeable pitch is used without a partition wall in order to lower power consumption and increase the volume of the byproducts which are to be compressed and evacuated, requires a larger rotor and pump housing than those pumps having a partition wall, which decreases efficiency.
In addition, in a dry vacuum pump as disclosed in the '463 patent, because bearings 110a, 110b supporting the pair of roots rotors 102, 103 and the pair of screw rotors 105, 106 are disposed at the intake side which changes between a vacuum and ambient pressure repeatedly during operation, grease for lubricating the bearings 110a, 110b can be leaked from the bearings 110a, 110b due to the pressure difference, which can cause trouble in the pump. Also, in a dry vacuum pump as disclosed in the '462 patent, because the bearings 110a, 110b are disposed on the intake side, the high temperature, pressure difference, friction and the like, can shorten the lives of the bearings 110a, 110b. 
Conventional dry vacuum pumps can comprise 4-5 roots rotors for lowering power consumption in operation, that is, for compressing more strongly the byproducts in gas state. The flow of the byproducts through the roots rotors is illustrated in FIG. 8. Even though a partition wall between the roots rotors is not illustrated in FIG. 8, it must be understood that a partition wall is formed between them in practice. However, because such conventional dry vacuum pumps using the aforementioned 4-5 roots rotors indispensably comprises rotor housings, paired rotors, partition walls, and the like, the assembly process is more difficult and complex. In addition, because the internal channels used to inhale and evacuate the byproducts are too long and complicated, gas leakage and internal accumulation of the byproducts is increased.